Water treatment to prevent embrittlement cracking



Nov. 16, 1948 w'. c. SCHROEDER ETAI. 2,454,253

um TREAT!!! 1'0 PREVENT mnlmmm' CRACKING Filed July 2, 1942 pH VALUEFIG. I

a 25m; CONCENTRATION, parts per milfion RELKI' ION OF H TO CONCENTRATIONFOR TYPICAL SALT OF WEAK KID AND STRONG BASE EFFECT OF.5ODIUM HYDROXlDEON OXIDATION OF STEEL BY WATER AT 590' F.

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( RELATIVE oxmmou NAOH, grams per liter 27 64 I25 2|s 343 I FIG. 2

W/L awe/v C. 55/120501? 4,, ABRAHAM A. BEEK INVENTORS ATTORNEY PatentedNov. 16,1948

UNITED STATES PATENT orFicE WATER TREATMENT TO PREVENT s EMBRITTLEMENTCRACKING Wilburn C. Schroeder, Calvert Hills, and Abraham A. Berk,Berwyn Heights, Md., assignors to the United States of. America asrepresented by the Secretary of the Interior Application July 2, 1942,Serial No. 449,454

(Granted under the act of March 3, 1883, as

amended April 30, 1928; 370 0. G. 757) 19 Claims.

This invention relates to the art of protecting.

boilers and the like against embrittlement crackand a few hundred partsper million of sodium silicate are necessary to cause cracking. The

. ing. More specifically, it relates to a method of dilute boiler watercan evaporate in small spaces in riveted seams or rolled in tube ends toattain a high concentration of sodium hydroxide such as has been foundto cause cracking in the laboratory.

It has been demonstrated that such concentrations can and actually dotake place. If the dilute boiler water very slowly leaks or cliifusesthrough a restriction toward a region of loweror atmospheric pressure,the water will tend to evaporate, both because of its superheat andbecause of the heat supplied from the boiler metal. This will leave aconcentrated solution in the region of the restriction. Our work hasdemonstrated that this process can concentrate a boiler water from 20 tomore than 500,000 parts per million of sodium hydroxide.

The behavoir of sodium hydroxide in boiler water and its action incausing embrittlement or intercrystalline cracking is more fullydisclosed in our Patent No. 2,283,954, granted May 26, 1942; in PatentNo. 2,283,955, granted May 26, 1942, to W. C. Schroeder; in ourcopending application, Ser. No; 237,982, nowPatent 2,297,670, grantedSeptember 29, 1942,- filed October 31, 1938, for Method of protectingboilers and the like against embrittlement; and also in Bulletin 443 ofthe Bureau of Mines, U. S. Department of the Interior, to whichreference may be had for a more comprehensive treatment of this subjectas well as for a, better and more complete understanding andappreciation of the features and advantages of the present invention.

Certain methods have been suggested in the past that it was believedwould eliminate embrittlement cracking in steam boilers. All of thesedepend on the maintenance in theboiler water of certain ratiosbetweenthe concentrations of an inhibitor and the boiler wateralkalinity, the

latter usually being due to the presence of sodium hydroxide and sodiumcarbonate. It will be seen,

therefore, that embrittlement cracking is caused by concentration of thesame substances which .produce the alkalinity essential to good boileroperation, and that these prior methods of boiler water treatment forthe prevention of embrittlement cracking left in the boiler water thesodium hydroxide which is the primary chemical which causesembrittlement cracking. Furthermore, these prior methods depend on theaddition to the boiler water of still other chemicals to arrest theaction of sodium hydroxide in producing embrittlement cracking. It ishighly desirable to avoid the presence in the boiler water of chemicalssuch as sodium carbonate and sodium hydroxide which can concentrate tocause embrittlement cracking, but it is also important to maintain therequisite alkalinity of the boiler water, as will be understood by thoseskilled in thisart.

Our invention provides for controlling and even for eliminating from theboiler water sodium hydroxide, as well as other dangerous alkalies, andin this way our invention provides for completely' eliminating alltendencies of the boiler water to produce embrittlement cracking, whileat the same time we provide for maintaining conditions in the boilerwater for satisfactorily preventing corrosion.

In the accompanying drawing:

Fig. 1 is a curve relating the pH value at room temperature to theconcentration at the equilibrium condition of a salt which may be usedin accordance'with this invention; and

Fig. 2 is a curve showing the effects of concentrating caustic soda incontact with steel, as a v I function of the sodium hydroxideconcentration, the concentrations being plotted on a cube root scale toadequately include the data.

In our investigation it has been found possible to control thealkalinity so that no matter how many concentrations of the boiler wateroccur,

the concentration of alkali in the water is 'lnsufllcient to causeembrittlement cracking.

Our work has disclosed that thermally stable salts of strong bases andweak acids in water solution have an alkaline reaction so that the hydrogen ion concentration in water solution is low enough so that waterscontaining these chemicals may have an alkalinity which makes themsatisfactory for boiler use, although there is no other cracking andalso to maintain the desired alboiler water.

kalinity for good boiler operation. Furthermore, higher concentrationsof the substances used for this purpose in accordance with thisinvention, such as will result from steaming or evaporation, will notsubstantially change the alkalinity of the boiler water as would be thecase if caustic soda were used to produce the initial alkalinity. Andmore important, we have found that the highest (concentrationsobtainable in solutions of the substances provided for this purpose byour invention will not cause embrittlement cracking of boiler steel.

These desirable objectives may be attained in accordance with thisinvention by simple and preferred modes of procedure as follows:

Example 1 (a) Any thermally stable salt of a strong base and a weak acidmay be selected, as on the basis of economy, desirability oravailability, and may be added to pure water, for example, until thedesired alkalinity is obtained. (b) The concentration so found may thenbe maintained in the (c) The absence from the water of free caustic,which could concentrate to cause embrittlement cracking, may beestablished in various analytical ways, as is well understood by thoseskilled in this art. (d) If free caustic is found to be present in theboiler water, it should be neutralized by the introduction of a suitableacidic substance until analysis shows that free caustic is no longerpresent, 1. e., until all of the alkali metal hydroxide in the water isat least stoichiometrically balanced by thermally stable weakly acidicradicals.

((1 Alternate) The amount of the selected salt to be added to pure waterto give the desired alkalinity in accordance with this invention, may

also be determined from a knowledge of the equilibria conditions forthis salt and pure water.

As is well known, water is not an inert matter and the process ofsolution of substances in water is often accompanied by chemicalreaction. The alkalinity of a solution of a salt of a strong base and aweak acid is recognized to be due to such an interaction with water.Thus:

BQH HA The double arrows are employed in their usual sense, i. e., theydenote a complete reversibility of the system. BA represents the salt ofa weak moved, the loss is made good by the dissociation of more wateruntil, eventually, a condition of equilibrium is established.

It will be noted from these equations that when the water is removedfrom a system containing only water and BA, only BA is formed and thereis no formation of-a strong base BOH, such as caustic soda. In thisspecification the term free caustic is used to denote any BOI-I inexcess of that derived from such hydrolysis and which may thereforeconcentrate to form a strongly alkaline solution when the water isremoved by steaming or evaporation.

The conditions governing hydrolytic equilibrium may be determined from aknowledge of the solubilities and ionization constants of the substancesinvolved. Thus, if the product of the concentrations of the ions H and Aexceeds that which can exist in the solvent, then some undissociated orinsoluble substance is formed. This disturbs the equilibrium of H and OHions, and a further dissociation of water occurs, until the ionicproduct of water is Just reached. A similar readjustment will take placewith respect to each undissociated or insoluble substance present in thewater.

The room temperature constants for the equilibria pertinent to mostsubstances present in boiler waters are well known. McKinneyCalculations for Corrections to Conductivity Measurernents for DissolvedGases, by D. S. Mc-

Kinney, Proc. American Society for Testing Materials; p. 1285, vol. 41(1941)) has shown how these several substances are /interrelated.Through the use of his method, it is possible to calculate theequilibria in dilute solutions such as are represented by boiler waters.For convenierice, all such calculations may be and herein are based onroom temperature conditions. From a knowledge of the concentrations ofdissolved substances, the hydrogen ion concentration can be computed.Conversely, the distribution of the electrolytes in solution can becalculated as a function of the hydrogen ion concentration. It I is,therefore, unnecessary to measure the alkalinity produced by a. selectedsalt of a strong acid and a weak base when the constants pertaining toits equilibria with water are known. The

I amount required to give to the water a desired alkalinity may becalculated and this concentration maintained in the boiler water.

Example 2 Another preferred mode of practicing this invention involvesthe determination of the pH value of the boiler water. (a) Any-thermallystable salt of a weak acid and a strong base may be selected and addedto pure water as previously described herein. The pH values atequilibrium conditions for any given number of concentrations of theselected salt in pure water may then be determined and a curve plottedrelating the concentrations to the pH values. A curve of this kind isillustrated in Figure 1. (11 Alternate) If desired, such a curve mayalso be calculated from the known constants. The points on such a curverepresenting pH values for optimum boiler operating conditions will alsorepresent the concentrations of the selected salt which will give thesepH values. In this manner, the range of concentrations of the selectedsalt to be maintained in the boiler water to give the desired pH rangemay be obtained, assuming the absence of free caustic from the boilerwater. For satisfactory operation and protection against corrosion andscale, the boiler water should be maintained at a pH value of 8.5 to 12and preferably at a pH value of 10 to 11.

(b') In using a curve of the kind'just described, the concentration ofthe selected salt, corrcsponding to the desired pH value on the curve,may be produced and maintained in the boiler water on the basis ofchemical analysis. As long as the pH value 101' the boiler water sotreated and the concentration therein oi the selected salt conform tothis curve, only the selected salt can be formed when the water isevaporated. In other words, under these conditions the selected salt maybe termed a "captive alkalizer,since on evaporation all the alkalizlngmetal is recaptured as the salt.

If any free caustic should be present in the boiler so that evaporationwould produce a concentrated solution of caustic soda, the pH of theboiler water would necessarily be above the curvev value correspondingto the concentration of the selected salt. If "free caustic is so foundto be present in the boiler water, it should be neutralized by theintroduction of a suitable acidic substance until the pH conforms to thecurve value. As indicated above, sodium carbonate is a source ofalkalinity. It is thus obvious that carbonic acid would not be asuitable acidic substance, since it would not eliminate free caustic byiorming therewith a salt stable at boiler temperatures. As indicated atpage 65 of Bulletin 443, referred to above, when trisodium phosphate isemployed as the captive alkalizer, an acid phosphate, as an alkalidihydrogen phosphate, isa suitable acidic substance. And as mentionedhereinafter, the secondary phosphate may be employed with the tertiaryphosphate. In either case, this is equivalent to addition with aquantity of the captive alkalizer (BA in the equilibrium equation above)of a quantity of the corresponding acid (HA therein), which by reactionwith concentratable NaOH (i. e. caustic in excess of the BOH of theequation) effectively eliminatesit from the system. When enoughneutralization is effected the pH will be reduced to the value of thecurve (Fig. 1); when some excess of the acidic substance is introduced,the pH of the boiler water will lie below the curve. To afiord completeprotection against embrittlement, the measured pH should never be abovethe value from the curve and when this condition prevails all of thealkali metal hydroxide in the water is at least stoichiometricallybalanced by thermally stable weakly acidic. radicals so that noconcentrated caustic can be formed when the water is evaporated. Asabove pointed out, boiler water containing as little as P. P. M. of freeNaOH supplied to a seam which leaks over a substantial period of time,can concentrate in time to more than 500,000 P. P. M. of NaOH.Therefore, for complete protection we prefer to always maintain azero-free caustic condition. However, as noted in Bulletin 443, abovecited, we were not able to producecracking of test specimens in testswhere the unconcentrated water contained 20 "P. P. M. of free NaOH orless, presumably because the loss of NaOH from the seams,.1or example ascarry-over in the vapor, tended then to become as great as the rate ofsupply of NaOH from the extremely dilute boiler solution. Thus, ourexperiments indicate that a very small concentration of free caustic canbe tolerated at least for short times without causing embrittlementcracking. This free caustic as a practical matter should never be morethan 20 parts per million expressed as sodium hydroxide or over 0.2pHunit above the curve value corresponding to the concentration of theselected salt when the curve value is between no substance will beformed which can cause embrittlement, but the selected salt is no longerbeing used-to full advantage to producethe ,desired alkalinity. II thepH-value is below the curve value corresponding to the concentrationbeing maintained of the selected salt, the desired alkalinity may berestored by adding caustic soda or other suitable alkaline substance, aswill be understood by those skilled in this art.

It will be seen from Equation 1 that so long as the pH value is notabove the curve value corresponding to the concentration of the selectedsalt, the caustic soda so added does not become free caustic such as onevaporation would form a concentrated solution which would causeembrittlement cracking, for the curve value- Other advantages -Animportant advantage of this method of conditioning water for boiler uselies in the fact that the total solids in water so treated are notmaterially increased bythe treatment, since no the concentration oftotal solids in the water so as to minimize carry-over of such solids inthe steam are materially decreased. There is, therefore, a resultanteconomy both from the energy bound up with the very hot water discardedand dltioning water for boiler use lies in the fact' that since solidsevaporated from such water contain no free caustic soda, such solidswhen deposited on turbine blades will not form a sticky deposit ofsodium hydroxide. Sincev the latter dep'osit acts as a binder for othersolids in the steam, it will be seen that in the absence of caustic,turbines can be run more efiiciently for longer periods withoutuneccnomical outages for cleaning.

-A further advantage of this method of conditioning water for boiler uselies in the fact that caustic soda concentrated in contact with hotmetal presents a serious corrosion hazard. Figure 2 (from Berl and VanTaack, Forschungsarbeltan auf dem Gebiete des Ingenieurwesens, 330,1930) shows how sodiiun hydroxide effects the oxidation of steel byreaction with water at 590 F. (the temperature of many modernhigh-pressure installations). Partridge and Hall (Attack on Steel andHigh Capacity Boilers as the Result of Overheating Due to SteamBlanketing,'by E. P. Partridge and R. E. Hall, American Society ofMechanical Engineers, trans, p. 597, 1939) have shown that even in themore modern boilers operated atthis temperature and pressure, the boilertubes may tend to run dry so that they contain steam rather than water.The boiler water, therefore, evaporates in contact with the hot steel ofthe tubes. When the water is treated with caustiesoda, this chemicalconcentrates on the tube surfaces and causes rapiddeterioration of themetal, resulting in serious corrosion and a consequent hazard to lifeandproperty. When the boiler water is' adjusted in accordance with thisinvention so that no concentrated solutions of caustic soda are formedif evaporation takes I place, no such corrosion hazard is presented intubes that attimes run dry.

Variations Any thermally stable salt .of a strong base and a weak acidthat has the property of increasing the pH of water without supplying toit free \caustic which would concentrate when the water of the saltsthat may be selected. However, we

prefer to use secondary and tertiary alkali phosphates, eitherseparately or together, notably secondary and tertiary sodiumphosphates, as these substances are very efiective. for the purpose'oithis invention and their use has been found to ive highly satisfactoryresults. to neutralize concentratable NaOH with the acid correspondingto the salt selected as the captive alkalizer, which in this instance isadded in the form of secondary phosphate. This is preferred since itaids in maintaining the selected conthat by maintaining such solids inspecified con-- centrations in boiler water containing no other sourceof alkalinity, definite conditions of pH are fixed so that when the pHis below such a fixed value, concentration of the boiler water will notresult in the formation of concentrated sodium hydroxide in the waterand no embrittlement cracking will result.

The terms strong and weak as applied to acids and bases herein and inthe claims, refer to the conductivity or the degree of ionization inaqueous solutions of moderate dilution. Strong bases and acids aredistinguished from weak ones by their extensive dissociation in watersolution, which is practically complete even at moderate concentrations.When a salt is said to be formed from a strong base and a weak acid,

-' the dissociation constant of the base is much greater than thedissociation constant of the acid. Mellor, "Modern Inorganic Chemistry,p. 372, Longmans, Green and Company, 1925, Getman and Daniels, Outlinesof Theoretical Chemistry, 6th edition, 1937, John Wiley and Sons.

The invention described herein, if patented, may be practiced,manufactured and used by or for the Government for governmental purposeswithout the payment to us of any royalty thereon.

It should be understood that the present disclosure is for the purposesof illustration only, and that this invention includes all modificationsand equivalents which are within the scope of the appended claims.

We claim as our invention and desire to secure by our patent:

1. A method of conditioning the water in a boiler to render it alkalinebut incapable of Thus we prefer yielding strong concentrations ofcaustic soda on evaporation, in which the alkalinity is imparted to thewater exclusively by hydrolytic dissociation of alkali-metal salts andespecially one of said salts selected from the group consisting ofsecondary and tertiary phosphates, silicates,.

borates, acetates and fluorides, and in which the alkalinity of theboiler water is controlled by determining the concentrations of all thealkalimetal salts of said group contained in the boiler water exclusiveof the selected salt and deter-'- mining the concentration of theselected salt which in conjunction therewith would impart to pure watera selected pH value desirable for boiler operation and lying between 8.5and 12, adding to the boiler water. the so determined concentration ofthe selected salt, testing the pH of the boiler water from time to time,and adding thereto such quantities of the selected salt and of acidand-basic agents as are indicated by the tests to be necessaryconcurrently to maintain the so determined concentration of the selectedsalt and an-alkalinity of the boiler water between the selected pH valueand 8.5.

2. A method according to claim 1, in which the selected pH value liesbetween 10 and 11.5.

3. A method according to claim 1, in which the selected salt istri-sodium phosphate.

4. A method according to claim 1, in which the selected pH value liesbetween 10 and 11.5 and in which the selected salt is tri-sodiumphosphate.

5. A method according to claim 1, in which the selected salt istri-sodium phosphate, and in which the adjustment of alkalinity iseffected in part at least by addition of an acid sodium phosphate,whereby the adjustment is effected concurrently with variation of theconcentration of sodium phosphate in solution, within the pH limits setforth.

6. The method of treating boiler water which consists in maintaining inthe boiler water a concentration of at least one thermally stable saltof a strong base and a relatively weak acid,

which salt concentration, if maintained in otherwise pure water, wouldimpart to the pure water a definite pH value lying between 8.5 and 12.testing the pH of the boiler water from time to time, and adding theretoa caustic-neutralizing acidic agent which forms a thermally stable saltwith the metal of the caustic to reduce its pH to said definite valuewhen the test shows its pH rising above said value.

7. The method of treating boiler water which consists in maintaining inthe boiler water a concentration of at least one thermally stable saltof a strong base and a relatively weak acid, which salt concentration,if maintained in otherwise pure water, would impart to the pure water adefinite pH value lying between 8.5 and 12, testing the pH of the boilerwater from time to time, adding thereto an alkaline substance to raiseits pH to said definite value when the test shows its pH to be droppingmaterially below said value, and adding thereto a caustic-neutralizingacidic agent which forms a thermally stable salt with the metal of thecaustic to reduce its pH to said definite value when the test shows itspH to be rising above said value.

8. The method of treating boiler water which consists in maintaining inthe boiler water a concentration of alkali metal phosphate which, itmaintained in otherwise pure water, would impart to the pure water adefinite pH value lying between 8.5 and 12, testing the pH of saidboiler water from time to time, adding caustic alkali thereto to raiseits pH to vsaid definite value when the test shows its pH to be droppingmaterially below said value, and adding thereto an acidic agent whichforms a thermally stable salt with the metal of the caustic to reduceits pH to said definite value when the test shows its pH to be risingabove said value.

9. The method of treating boiler water which consists in maintaining inthe boiler water a concentration of tri-sodium phosphate which, ifmaintained in otherwise pure water, would impart to the pure water adefinite pH value lying between 8.5 and 12, testing the pH of saidboiler water from time to time, adding sodium hydroxide thereto to raiseits pH to said definite value when the test shows its pH to be droppingmaterially below said value, and adding thereto an acidic agent whichforms a thermally stable salt with the metal of the caustic to' reduceits pH "consists in maintaining in the boiler water a concentration of athermally stable alkali metal salt of a relatively weak acid, which saltconcentration, if maintained in otherwise pure water, would impart tothe pure water a de f1nite pH value lying between 8.5 and 12, testingthe pH of the boiler water from time to time, adding thereto an alkalisubstance to raise its pH to said definite value when the test shows itspH to be dropping materially below said value, and adding thereto acaustic-neutralizing acidic agent which forms a thermally stable saltwith the metal of the caustic to reduce its pH to said definite valuewhen the test shows its pH to be rising above said value. A r

12. A method according to claim 6 in which the added acidic agentcomprises an acid which combines with free caustic to form the firstmentioned thermally stable salt, so that its addition concurrentlyeliminates free caustic and aids in maintaining the concentration ofsaid salt.

13. The process of protecting boiler metal from caustic attack whichcomprises employing as the boiler water a solution consistingessentially of tri-sodium phosphate and water, testing the solution todetermine whether it contains free hydroxide of an alkali metal inexcess of that ofa pure aqueous solution of tri-sodium phosphate ofequal phosphate concentration, and adding to the boilerwater solution asneeded a suificient quantity of an-acid phosphate to neutralize saidfree hydroxide.

14. The process of protecting boiler metal from caustic attack whichcomprises operating the boiler with a boiler water solution consistingessentially of tri-sodium phosphate and water, testing the boiler watersolution to determine whether it contains tree hydroxide of an alkalimetal in excess of that of a pure aqueous solution of tri-sodiumphosphate of equal phosphate concentration, and adding to the boilerwater solution as needed as sumcient quantity of an acidic substance toneutralize said tree bydroxide.

15. The process of protecting boiler metal from caustic attack whichcomprises operating the boiler with a boiler water solution consistingessentially of water alkalized with a thermally 10 stable salt of astrong base and a relatively weak acid, testing the solution todetermine whether it contains free hydroxide of an alkali metal inexcess of that of a pure aqueous solution of said salt of equal acidradical concentration, and adding to the boiler water solution as neededa sufiicient quantity of an acidic substance to neutralize said'freehydroxide.

16. The method of protecting boiler metal from caustic attack whichcomprises operating the boiled with a boiler water consistingessentially oi an aqueous solution of tri-sodium phosphate whichcontains no free hydroxide of an alkali metal in excess of that of apure aqueous solution of said phosphate of'equal phosphateconcentration.

17. The method of protecting boiler metal from caustic attack whichcomprises operating the boiler with a boiler water consistingessentially of an aqueous solution of a thermally stable salt of astrong base and a weak acid which contains no free hydroxide of analkali metal in excess of that of a pure aqueous solution of said saltof equal acid radical concentration.

18. The method of protecting boiler metal from caustic attack whichcomprises operating the boiler with boiler water consisting essentiallyof an aqueous solution of tri-sodium phosphate including a smallproportion of disodium phosphate.

19. The method of protecting boiler metal from caustic attack whichcomprises operating the boiler with boiler water consisting essentiallyof an aqueous solution of a thermally stable salt of a strong base and aweak acid including a small proportion of a-thermally stable causticneutralizing acidic agent.

WILBURN C. SCHROEDER. ABRAHAM A. BERK.

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